Cement-based concrete remains the most widely employed material in construction. Despite the existence of solutions such as incorporation of metal reinforcements, there always exists a need to improve the properties of concretes, whether their mechanical strength, their resistance to aging or the control of the process of hydration of the cement forming the basis of concretes.
It has been demonstrated in preceding studies that the incorporation of carbon nanotubes in cements exhibits numerous advantages. This is because carbon nanotubes (or CNTs) confer improved mechanical properties and electrical and/or thermal conduction properties on any composite material in which they are present; in particular, their good mechanical properties and in particular their good properties of resistance to elongation are related in part to their very high aspect (length/diameter) ratios.
By way of example, in the document US 2008/0134942, the addition of carbon nanotubes at a content of greater than 0.2%, combined with the addition of small contents of a plasticizer, makes it possible to reinforce cements in terms of resistance to compression and to deformation.
The document WO 2009/099640 describes a method for the preparation of materials based on reinforced cement which consists in dispersing carbon nanotubes in a surfactant solution, in a surfactant/CNTs ratio of between 1.5 and 8, using ultrasound, and then mixing the dispersion with a cement, so as to obtain a material comprising 0.02% to 0.1% of carbon nanotubes with respect to the cement. The carbon nanotubes employed preferably have a diameter ranging from 20 to 40 nm and a length ranging from 10 to 100 μm. The surfactants are preferably polycarboxylate-based superplasticizers. The CNT dispersion comprises more than 98% of water and a low content of superplasticizer, generally of less than 1%. This dispersion is generally used rapidly after its preparation and is not stored. According to this document, the quality of the dispersion of the CNTs within the material results from the quality of the dispersion of the CNTs in the surfactant solution obtained by ultrasound. The effects obtained are the increase in the Young's modulus and in the bending strength and also a reduction in the phenomenon of endogenous shrinkage.
Similar results with regard to the effects of carbon nanotubes as cement reinforcer are described in the document Cements & Concretes Composites, 32 (2010), 110-150.
According to the document Materials Science and Engineering A, 527, (2010) 1063-1067, the mechanical reinforcement resulting from the presence of the carbon nanotubes is also accompanied by the densification of the cement.
Pervushin et al. have presented, at the Nano-technology for green and sustainable construction international conference, 14-17 Mar. 2010, in Cairo, Egypt, the results obtained on the reinforcing of cement by virtue of the incorporation of carbon nanotubes at contents as low as 0.006% with respect to the cement, in the form of an aqueous dispersion obtained by hydrodynamic cavitation starting from powdered CNTs and a superplasticizer. However, this study shows that these CNT dispersions are not stable over time and thus have to be used rapidly for the cement reinforcing application; in addition, as the CNTs are generally provided in the form of agglomerated powder grains, the mean dimensions of which are of the order of a few hundred microns, the handling thereof may present safety problems due to their pulverulent nature and their ability to generate fines in the plants where they are used.
In patent application WO 2012/085445, provision has been made to introduce, into the curable inorganic system, carbon nanotubes not in the powder form but in the form of a master batch of carbon nanotubes comprising a polymer binder. The process consists in preparing a dispersion in water of carbon-based nanofillers starting from a master batch of carbon-based nanofillers and of a polymer binder, in the presence of at least one superplasticizer, and in subjecting this dispersion to a treatment by high-speed mixing, for example by sonication, by cavitation of the fluids or using a Silverson high shear mixer or a bead mill. The dispersion is introduced, as is or rediluted, into a curable inorganic system, such as a cement, to ensure a final content of carbon-based nanofillers ranging from 0.001% to 0.02% by weight, preferably from 0.005% to 0.01%, with respect to the curable inorganic system. According to this process, the procedure for the dispersion still remains lengthy and difficult to carry out on a greater scale, and the composite material obtained, such as a concrete, comprises a low content of a polymer binder, which may possibly affect the properties thereof.
Consequently, the introduction of carbon nanotubes into cement-based materials or any other curable inorganic system still raises a few negative points which have to be improved.
It is therefore desirable to have available a means which makes it possible to simply and homogeneously distribute carbon nanotubes within a cement-based material or any other curable inorganic system for the purpose of preparing composite materials of high mechanical strength and preventing the cracks resulting from the aging of these materials.
Furthermore, due to their pulverulent nature and their ability to generate fines in manufacturing plants, it is preferable to be able to work with CNTs in agglomerated solid form of macroscopic size.
The applicant company has discovered that these requirements could be met by introducing carbon nanotubes into cement-based materials or any other curable inorganic system via a master batch based on carbon nanotubes and on a superplasticizer. This is because the use of a superplasticizer is always recommended in order to increase the compactness and the mechanical strength of concretes and mortars and to improve their fluidity and their handling.
The present invention thus consists in replacing the superplasticizer with a superplasticizer doped with carbon nanotubes in the existing manufacturing processes and devices of the building and construction industry and also in the oil field.
The process of the introduction of the carbon nanotubes according to the present invention is simple, fast and easy to carry out from an industrial viewpoint while observing the constraints of health and safety. It does not require modifying the conventional processes for the manufacture of composite materials based on curable inorganic systems which already use a superplasticizer as high water-reducing dispersing additive, while resulting in materials which are denser and mechanically reinforced.
Furthermore, it is apparent to the applicant company that this invention can also be applied to other carbon-based nanofillers than carbon nanotubes and in particular to carbon nanofibers and to graphenes.